This invention relates to the construction of improved masks used in the generation of patterns employed in the production of integrated circuits. In particular, this invention relates specifically to a self-aligning mask used in a multi-layer ceramic (MLC) device.
An MLC is a multi-layer ceramic packaging substrate. Numerous layers, in the order of 20-30 are employed, each made of a ceramic material and having an electrically conductive personalized wiring pattern. The layers are stacked to define within the stack power layers, signal layers, redistribution layers and the like all interconnected.
A large number of VLSI chips are mounted on the top of the substrate which in turn establishes all electrical connections to and between those chips. On the top portion of the substrate, each chip site has a central array of contact pads, I/O connections and the like surrounded by one or more frames of engineering pads. Such MLC devices are described in greater detail in: A. J. Blodgett and D. R. Barbour-"Thermal Conduction Module: A High-Performance Multilayer Ceramic Package". I.B.M. Journal of Research and Development, Vol. 26, No. 1, January 1982; G. A. Walker, "Failure Analysis of Multilayer Ceramic Substrates", 18th Annual Proceedings, Reliability Physics, 1980 IEEE Catalog No. 80CH1531-3.
The MLC ceramic is a fired Al.sub.2 O.sub.3 substrate having on its top layer the above-described features utilizing molybdenum metallurgy. Following firing, additional metallurgy is required on the Mo features to provide chip, I/O, pin and flange joining. Currently, metal plating is used to define this metallurgy. Such plating techniques constitute a major process problem in the fabrication of the complete MLC. Plating may, for example, be extraneous and located where it is not needed. It may bridge between features or conversely not be present and not plate certain features. Bath chemistry and control for accurate plating is complex. For example, thickness control, normally the time in the bath is difficult to accurately replicate. The effect of impurities co-deposited both in terms of bulk and surface is a serious problem. Finally, there is a severe environmental impact on the dumping and disposal of used baths. Cost considerations, processing difficulties and the introduction of defects in the product also detract from the overall yield of the final device. Given the extensive processing (many months) needed to reach these final steps, processing which detracts from the overall yield rate must therefore be minimized. Moreover, separate masking steps are not always accurately aligned with the existing metalization so that errors occur in the plating step requiring extensive operator time to correct. Accordingly, there exists a need to define an improved technique for acceptable metallurgical definition following firing of the MLC product.
The prior art is replete with a host of proposals for using various masks and alignment systems in conjunction with X-ray lithography techniques. Typical are various systems proposed in U.S. Pat. Nos. 3,740,280; 3,742,229; 3,742,230; 3,743,842; 3,767,398; 4,022,927; and 4,035,522. Reference is made to U.S. Pat. No. 4,018,938 defining a method of constructing aligned masks utilizing X-ray illumination through the back side of the substrate. On top of an intermediate product comprising a substrate 10 having a thin plating layer 12 with plated areas 17, an additional resist is coated over the entire surface. The substrate is then illuminated by X-rays having a wavelength of approximately 8.3 .ANG. to expose portions of the second resist which are not protected by the plated areas. Those plated areas modulate the X-ray illumination, i.e., serving as the mask itself to avoid misregistration. The resist is then developed such that the unexposed areas leave holes directly above the plated areas. A second plating operation is then carried out to increase the height of the plated areas and both resist layers are removed leaving a final product having a suitable high aspect ratio.
Reference is also made to U.S. Pat. No. 4,215,192 which is also directed to X-ray lithography utilizing a mask membrane having absorbers interposed between the X-ray source and a resist coated on top of a substrate for processing. A reduction in exposure time is achieved by utilizing tungsten as a target to permit operation at higher power densities. While offering advantages in terms of processing times, the system does not achieve the necessary self-aligning for use in MLC products having an extensive highly dense array of metalization at the top layers.
The prior art also includes a number of different proposals for resist materials for use in X-ray lithography systems. Typical are various resist processing techniques as set forth in IBM TDB, Vol. 18, No. 7, pp. 2344-2347, December 1975; IBM TDB, Vol. 19, No. 11, p. 4193, April 1977; Taylor "X-Ray Resist Materials", Solid State Technology, May 1980, page 73; U.S. Pat. No. 4,061,829; and Haller et al, "High Resolution Positive Resist for Electron Beam Exposure", IBM Journal, pp. 251-256, May 1968.
Accordingly, while the prior art offers a number of X-ray lithography processing techniques and a host of resist materials of choice, there exists a need in this technology to define a process which is self-aligning in the context of a highly complex molybdenum metallurgy pattern on the top surface of an Al.sub.2 O.sub.3 ceramic substrate. Such a self-aligning mask would remove a significant processing limitation in eliminating the plating requirement for final metalization in the formulation of a complete MLC.